Measurement of flow fractions, flow velocities, and flow rates of a multiphase fluid using NMR sensing

ABSTRACT

Various flowmeters (10, 70, 80) each have at least one NMR sensor (12, 72, 82) for measuring the various flow parameters of a flowing fluid. The NMR sensor may be used to provide a gradient magnetic field, which provides separate velocities when the fluid is multiphase (FIG. 10). The flowmeters may also be used to measure flow rate directly. The flowmeters may also be used to determine the separate flow fractions of a multiphase fluid.

RELATED PATENT APPLICATION

This application claims the benefit of U.S. Provisional Application Ser.No. 60/050,637, filed Jun. 24, 1997 and entitled "Measurement ofSeparate Flow Fractions and Flow Rates of a Multiphase Fluid."

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to instruments for meteringmultiphase fluids, that is, measuring the separate percentages of gasand liquid (or separate liquids such as oil and water) in such fluids aswell as the velocity and rate of the mixed fluid or the separate liquidand gas velocities and rates.

BACKGROUND OF THE INVENTION

A "flowmeter" is a device used to measure the flow of fluid materialthrough a defined area, such as the cross sectional area of a pipeline.The "flow" is the motion of the fluid, which may be liquid or gas or acombination of both. The flowmeter measures the flow in terms of theflow rate, the amount of the fluid that flows over a given period.

There are various ways to express flow rate, such as by mass flow rate,volumetric flow rate, or velocity of flow. Mass flow rate is the flowrate in units of mass per unit time, i.e., kilograms per second.Volumetric flow rate is the flow rate in units of volume per unit time,i.e., cubic meters per second. Velocity of flow is in units of lengthper unit time, i.e., meters per second.

Most measurements of flow are in terms of volumetric flow rate. Massflow rate can be calculated from this measurement, although variationsin pressure, density, temperature must be taken into account,particularly for gases. Similarly, a measurement of velocity of flow canbe used to calculate mass or volumetric flow rate.

Conventional flowmeters are designed for single phase fluids, that is,fluids that are either gas or liquid. Some existing flowmeters aremechanical, where the flowing fluid displaces or rotates a solid body.The displacement or rotation is proportional to the flow rate. Anothertype of flowmeter is a differential pressure flowmeter, in which fluidis forced through some type of restricted area. This causes its velocityto change, causing a pressure difference that is proportional to flow.By measuring the pressure difference, such as with a differentialpressure transducer, the flow rate can be determined. Other flowmetertypes are thermal, electromagnetic, vortex generating, and ultrasonic.The criolis force flowmeter is widely used for measuring mass flow.

When the fluid whose flow rate is to be measured is a multiphase fluid,special problems arise. An example of such a fluid is a hydrocarbonfluid, which is typically a mixture of oil and gas as well as water. Fora multiphase fluid, there is often a need to know the liquid and gas"cuts", that is, the fractional amount of each constituent at a givenpoint in a flowline, as well as their rates. In the case of petroleumfluids, there is a need to know the oil cut as distinguished from boththe water and gas cuts.

One consideration when measuring the flow rate of multiphase liquids isthat the gas component tends to flow at a higher velocity than theliquid component. It is therefore necessary to separately measure thegas and liquid flow velocities or to measure the total flow velocityafter mixing the fluid.

An additional consideration in measuring a multiphase fluid is that thedensity of the gas, except at very high pressures, is low compared tothat of the liquid. This makes direct measurement of the gas fractiondifficult. Typically, the liquid fraction is measured and the remainderis assumed to be gas. In other words, if a pipe section is half filledwith liquid, then the other half is assumed to be gas.

The conventional approach to measuring multiphase flow rates is toseparate the fluid into its constituents. This permits conventionalsingle phase metering techniques. However, especially in the petroleumindustry, as the water and gas content of recoverable petroleum outputhas increased and oil fields have become more inaccessible, there is aneed for more sophisticated multiphase flowmetering equipment.

Several patents have been issued that describe the use of nuclearmagnetic resonance (NMR) analysis to analyze fluid flows that are notnecessarily multiphase.

These include U.S. Pat. No. 4,531,093, to Rollwitz, et al., entitled"Method and Apparatus for Coal Analysis and Flow Measurement"; U.S. Pat.No. 4,536,711, to King, et al., entitled "Method and Apparatus forMeasuring Flow in a Pipe or Conduit"; and U.S. Pat. No. 4,866,385, toReichwein, entitled "Consistency Measuring Device".

NMR techniques have been specifically applied to analyzing multiphasefluids. U.S. Pat. No. 4,785,245, entitled "Rapid Pulse NMR Cut Meter,"describes a flowmeter that uses NMR analysis to determine the fractionof one component of a multiphase fluid flowing in a pipeline. Theamplitude of the NMR signal from a desired component is compared to areference signal representing a 100% sample of the component.

SUMMARY OF THE INVENTION

One aspect of the invention is a flowmeter having an NMR sensor and aprocessor that is programmed to determine flow velocity. Thecalculations are based on the time it takes the sensor coil tocompletely fill with freshly polarized fluid. When filled with freshfluid, the NMR signal amplitude is a maximum. When the sensor ispermitted to fill only partially with fresh fluid, the signal amplitudeis smaller. The difference between the two signals can be used todetermine flow velocity.

Various other embodiments of the invention uses NMR sensing to determineflow rates directly, and to determine separate flow velocities formultiphase fluids. A flowmeter having dual NMR sensors can be used todetermine separate flow fractions of a multiphase fluid.

An advantage of the invention is that each fraction of a flowingmulti-phase fluid can determined. Depending on which of the varioustechniques are used, total or separate flow velocities for each flowfraction can be measured, as well as the totalized and fractional flowrate.

The sensor equipment can be located separately from the detectionelectronics and processing equipment. The results are available in realtime as the fluid is flowing in situ.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a flowmeter having an NMR sensor and ESR sensor formeasuring flow fractions and a venturi for measuring flow velocity.

FIGS. 1A and 1B illustrate the NMR sensor and ESR sensor, respectively,of FIG. 1.

FIGS. 2-5 illustrate two embodiments of the pre- polarizer of FIG. 1.

FIG. 6 illustrates the magnetic field configuration provided by thepre-polarizer and the NMR and ESR sensors of FIG. 1.

FIG. 7 illustrates a flowmeter having an NMR sensor and ESR sensor formeasuring flow fractions and flow velocity.

FIGS. 8A and 8B illustrate how velocity may be measuring using the NMRsensor of FIG. 7.

FIG. 9 illustrates a flowmeter that uses two NMR sensors for measuringflow fractions.

FIG. 10 illustrates how a gradient polarization field and an NMR sensormay be used to determine separate liquid and gas flow velocities.

FIG. 11 illustrates a flowmeter having only an ESR sensor for measuringan oil fraction and oil velocity.

FIG. 12 illustrates a flow meter having an ESR sensor and gamma raysensor for measuring flow fractions and flow velocities.

DETAILED DESCRIPTION OF THE INVENTION

The following description describes several different flowmeters, eachof which provides NMR (nuclear magnetic resonance) or ESR (electronicspin resonance) signals. In some cases, the flowmeter provides bothtypes of signals. ESR is sometimes referred to as EPR (electronparamagnetic resonance). ESR is similar in its principal of operation toNMR but unpaired electrons (due to free radicals, broken bonds, andparamagnetic elements) in materials are detected instead of the nucleiof the hydrogen atom.

The flowmeters analyze these signals to derive the fractional amounts ofliquid and gas (or fractional amounts of oil, water, and gas) in amultiphase fluid. Various methods of using these flowmeters to alsoderive flow velocity are also described. The flow velocity may be ameasurement of a mixed fluid, where the gas and liquid flow at the samevelocity, or for separate velocities of the gas and liquid. Methods ofderiving total and fractional flow rates are also described.

In the examples of this description, the multiphase fluid is measured asit is transported in a pipeline. However, the flowmeters describedherein can be used in conjunction with other fluid transport means suchas a conveyor belt, an open trough, a vertical flow under the force ofgravity, or by any other transport means that moves the fluid throughspace as a function of time.

For purposes of example, the multiphase fluid is assumed to be acombination of hydrocarbon gas and oil as well as water. Thus, the fluidhas two liquid constituents (oil and water) and a gas constituent.Various flowmeters described herein are capable of measuring thefraction of each constituent. However, the flowmeters are useful forother multiphase fluids, with another example being a gas that ispartially liquefied. The invention can also be used with single phaseliquids or gases.

For NMR sensing, the nuclei to be detected are those of hydrogen. Thesenuclei are present in water, oil, and hydrocarbon gases, and providebetter NMR measurement sensitivity than other types of atomic nuclei.

Flowmeter with NMR Sensor, ESR Sensor, Fractional Analysis, and VenturiVelocity Measurement

FIG. 1 illustrates a first embodiment of the invention, an NMR/ESRflowmeter 10, which measures fractional amounts of liquid and gas in amultiphase fluid. In the example of this description, the fluid containswater, oil, and gas. As explained below, flowmeter 10 uses both NMR(nuclear magnetic resonance) and ESR (electronic spin resonance)analysis.

The fluid material passes through a pre-polarizer 11, which isessentially a magnet that polarizes the magnetic moments of both nucleiand free or un-paired electrons that are present in the fluid. Suchpre-polarization provides rapid and sensitive detection of NMR signalsfrom a flowing material, especially from materials that have a longpolarization time, i.e., long T₁ materials. For ESR, extended lengthpre-polarization is not needed.

The amount of polarization provided by pre-polarizer 11 depends onwhether NMR or ESR sensing is being performed, on the properties of thefluid, and on the flow velocity. For a given fluid, the polarizationtime is a certain time constant. For ESR signals, the polarization timemy be less than a microsecond. For NMR signals, the polarization timemay be as much as several seconds or more.

For NMR sensing, the polarization time constant, T₁, is the spin-latticerelaxation time. For hydrogen nuclei (protons) in water, T₁ is in arange of 2.0 to 2.5 seconds. An exception is if the water iscontaminated with paramagnetic ions. For crude oil, T₁ varies withtemperature but typically has two values, one in the range of 0.05 to0.15 seconds and one in the range of 0.4 to 0.9 seconds. These are dueto the different molecular constituents and the oils may have more thantwo values of T₁. For hydrocarbon gases, T₁ varies with temperature,pressure, and the type of gas. For hydrogen gas, T₁ can be quite short,from 10's to 100's of microseconds. For methane, T₁ is from 10's to100's of milliseconds at typical land pipeline temperatures andpressures, and is up to a few seconds at subsea temperatures andpressures. For heavier hydrocarbon gases, T₁ typically ranges fromhundreds of milliseconds to several seconds.

The degree of magnetic polarization, M_(p), prior to measurement of theNMR and ESR signals determines the amplitude of these signals. For asimple material, the magnetic polarization may be expressed as ##EQU1##where M₀ is the maximum polarization in a magnetic field of strength H₀,t is the time the material is exposed to H₀, and T₁ is the spin-latticerelaxation time constant. The polarization time, t, is equal to thelength of the pre-polarization magnet divided by the flow velocity.

As examples of the above calculation of the magnetic polarization, fort=T₁ then M_(p) =0.632 M₀, and the detected NMR signal amplitude is36.78% lower than the maximum that would be attained if t>>T₁. For M_(p)=0.9 M₀ then t=2.3 T₁, for 0.95 then t=T₁ and for 0.99 then t=4.6 T₁.For NMR measurements, the amplitude of the NMR signal from material of agiven type and concentration is preferably substantially constantdespite changes in flow velocity. To this end, the length of thepolarization magnet should be such that the polarization times at thehighest velocity and at the lowest velocity are in accordance with anallowable variation in M_(p) /M₀ as determined by Equation (1). Theinvention achieves this goal with a compact and cost effective design.Alternatively, if velocity is known, flowmeter 10 can provide less thanfull polarization and compensation can be made for the effect ofvelocity changes on the NMR signal amplitude.

FIGS. 1A and 1B illustrate the NMR sensor 12 and ESR sensor 13,respectively, of FIG. 1. NMR sensor 12 and ESR sensor 13 each have amagnet 12a and 13a, which provides an H₀ field through the flow pipe inthe sensor 12 and 13, and a sensor coil 12c and 13c. Each also has atuning element (which may be a resonant cavity), that is tuned to theNMR or ESR frequency, as appropriate, illustrated as a tuned capacitor12b for NMR sensor 12 and a resonator or tuned coil 13b for ESR sensor13. The flow conduits through sensors 12 and 13 are of non-conductivematerial to permit passage of the RF fields from the sensor coil 12c and13c (or cavity or other resonator) to the flowing fluid.

FIGS. 2-5 illustrate two different embodiments of pre-polarizer 11 usingpermanent pre-polarizer magnets 11b. FIGS. 2 and 3 are a plan view and across sectional view of a pre-polarizer 11 having a round flow channel11a. FIGS. 4 and 5 are a plan view and a cross sectional view of apre-polarizer 11 having a rectangular flow channel 11a. In bothembodiments, the flow channel 11a is folded in a single plane betweentwo steel or iron plates, which are the pole pieces of the magnet.Permanent magnets 11b are located along both sides of the plane toprovide the magnetic field to the pole pieces. The conduit could also beon parallel planes between these pole pieces. These embodiments ofpre-polarizer 11 are designed to provide optimum polarization with agiven amount of magnetic energy and to minimize the size ofpre-polarizer 11. A conventional straight flow conduit between the polesof a polarizing magnet or a spiral flow conduit between the poles of apolarizing magnet could also be used. The flow conduit 11a in thepre-polarizer is of non-ferromagnetic material that permits externalmagnetic fields to penetrate through the flowing fluid.

FIG. 6 illustrates the magnetic fields provided by pre-polarizer 11 aswell as by the sensor magnets in the NMR sensor 12 and ESR sensor 13.Separate sensor magnets of different Ho values can be used in the NMRand ESR sensors. The flow channel is transverse between the magnetpoles, N and S. A magnet based on a Watson configuration, as illustratedin FIGS. 2-5, is especially suitable although other magnet designs canbe used.

Referring again to FIG. 1, pre-polarizer 11 incorporates features toprovide uniform mixing of the gas and liquid within the fluid. Thisresults in a single flow velocity for the mixed fluid, which is measuredwith venturi 14.

After polarization, NMR sensor 12 is used to detect the NMR signal. NMRsensor 12 is comprised of a magnet, which provides a static field ofintensity, H₀, and a radio frequency (RF) coil, L, which is tuned bycapacitance to the NMR (Larmor) frequency, f₀. This coil encircles (oris in close proximity to) the flowing fluid and produces an RF magneticfield that is oriented normal to the static field, H₀. The NMR frequencyis given by

    f.sub.0 =γH.sub.0 MHz,                               (2)

where H₀ is in Tesla and γ is the gyromagnetic moment of the nucleiunder observation. For hydrogen, this frequency is approximately 42.6 H₀MHZ. The ESR sensor 13 used to detect ESR signals from crude oil andother ESR-favorable materials is comprised of a magnet that provides astatic field of intensity H₀ and a coil or cavity resonator tuned to theESR frequency.

NMR sensor 12 and ESR sensor 13 are each in communication with anassociated detector 12a and 13a, respectively. Detectors 12a or 13a areeach comprised of a signal generator for energizing the sensor coil, orresonator, at the NMR or ESR frequency, as well as electronic circuitryfor receiving, amplifying, and detecting an output signal from thesensor and delivering the output signal to processor 19. For simplicityof description, the combination of a sensor and detector for NMR or ESRis sometimes referred to herein as simply a "sensor". However, anadvantage of the invention is that the sensor (comprised of the tunedcoil and the sensor magnet) and the detector may be separate and remotefrom each other.

According to NMR theory, the polarized fluid will emit an RFelectromagnetic field oscillating at the NMR (Larmor) frequency whenenergized by a pulse (such as a 90 degree pulse) of RF electromagneticfield of the NMR (Larmor) frequency. When the NMR sensor coil isenergized by a multiplicity of such pulses, nuclei in the fluid beingmeasured produce sinusoidal electrical signals at the NMR frequency inthe sensor coil as one or more RF pulses. The duty period of the pulsesand the delay between pulses may be controlled by the sensor'selectronic circuitry to maximize a desired NMR response.

For one use of the embodiment of FIG. 1, a single pulse may be used toenergize the sensor coil. The NMR output signal is a transient freeinduction decay (FID) signal from the hydrogen nuclei. For maximumamplitude of the FID signal, the nuclei should be fully pre-polarizedprior to the pulse and the pulse energy should be such as to deflect thenuclei by an angle of 90° from alignment with the static field, H₀. Apulse that deflects the nuclei in this manner is referred to as a "90°pulse". However, useful NMR FID signals may be obtained with less thanfull polarization and with a pulse that is shorter than 90°.

The peak amplitude of the FID signal is proportional to the total numberof hydrogen nuclei in the coil of NMR sensor 12, to the amount ofpolarization, M_(p), and to the energy of the RF pulse. Using NMRanalysis techniques, the total hydrogen density of the fluid can becalculated. For a fluid containing oil, water, and gas, this totalrepresents the contribution of each constituent.

Processor 19 is programmed to calculate total hydrogen density of thefluid from the FID signal data. It has associated memory for storing theprogramming, calibration factors, and data used for such calculations.It may also be programmed to perform various other calculationsdescribed below, such as the separate fractions for gas, water, and oil,as well as total velocity and flow rates of the fluid.

The NMR measurement of the total hydrogen density, together withpressure and temperature measurements from sensors 15 and 17, can beused to determine the gas fraction in the fluid. Specifically, if thesensor coil were to contain only liquid, the FID signal would have aknown maximum amplitude value. This value decreases with the presence ofgas. Thus, the gas fraction may be calculated as a function of the FIDsignal amplitude and the fluid pressure and temperature.

ESR sensor 13 measures the fraction of crude oil in the fluid (the "oilcut"). Because of the short polarization time for ESR signals, ESRsensor 13 need only be a few centimeters long. For ESR signals, thefrequency, f₀, is approximately 28 H₀ GHZ. Thus, for a given magneticfield, ESR resonances are much higher in frequency than NMR resonancesand the ESR sensitivity is greater.

ESR sensor 13 senses unpaired electrons, which are present in most crudeoils, that is, petroleum liquid prior to refining. No ESR signal isproduced by water or gas. The ESR signal amplitude is proportional tothe density of the unpaired electrons, and thus to the amount of oil ina pipe cross section. Thus, by using ESR to sense the oil in amultiphase flow, a separate and direct measurement of the crude oilfraction can be obtained. Although not all crude oils have the samesignal amplitude for a given concentration, calibration factors for thetype of oil can be readily derived and applied. In addition, certain ESRsignal features in crude oils can provide the basis for identifyingcertain crude oils or detecting particular constituents.

Once the oil fraction and gas fraction are known, using the methodsdescribed above, the remainder of the fluid can be assumed to be water.Alternatively, the water fraction can be determined by the NMR sensor asdescribed below.

In sum, in the case of flowmeter 10, NMR sensor 12 is used to determinethe total fluid density. ESR sensor 13 is used to determine the oilfraction. A mixer is incorporated in pre-polarizer 11 to insure ahomogeneous mixture of gas and liquid that flows at a single velocity.The total flow velocity is measured in venturi 14 using differentialpressure, total pressure, and temperature measurements. FIG. 1illustrates suitable locations for a temperature sensor 15, pressuresensor 16, and differential pressure sensor 17.

Flowmeter with NMR Sensor, ESR Sensor, Fractional Analysis, and NMRVelocity Analysis

FIG. 7 illustrates an alternative embodiment of the invention, aflowmeter 70 where the fluid is not mixed and a venturi is not used.Flowmeter 70 has a pre-polarizer 71, NMR sensor 72, ESR sensor 73, andtemperature and pressure sensors 75 and 77. Each of these componentsoperates in a manner similar to the corresponding components offlowmeter 10.

For determining liquid and gas fractions, flowmeter 70 may be programmedand used in the manner described above for flowmeter 10. Specifically,NMR sensor 72 and ESR sensor 73 may be used to provide signals that areanalyzed by processor 74 to determine separate oil, water, and gasfractions.

Flow velocity is obtained using NMR sensor 72, energized by thefollowing "variable delay" pulse sequence:

    p.sub.1 -τ-p.sub.2 -acquisition-d,

where τ is a variable delay between pulses. It is assumed that a lengthof the pipe line, X₁ carrying the fluid prior to entry into NMR sensor72 is immersed in a static magnetic field to pre-polarize the nuclei,such as is accomplished with pre-polarizer 71. The length of the pipeline inside the NMR sensor coil is X₀, where X₁ >>X₀. If the fluid is aliquid (perhaps an oil-water mixture) flowing at velocity, v, then thedelay time, τ, is started at 1 millisecond and increased to a value thatis longer than X₀ /v. It is also assumed that the static magnetic fieldis in the z direction so that all the proton nuclear magnetization arealigned or partially aligned along the z direction before entering thecoil of NMR sensor 72.

For velocity measurement, the FID signal is observed immediately afterthe first 90 degree RF pulse, P₁. Under these conditions, the FID signalamplitude may be modeled as follows:

    F=M.sub.0 dSX.sub.0 (1-e.sup.(-X.sbsp.1.sup./vT.sbsp.1.sup.))f,(3)

where S is the cross sectional area of the pipe, d is the density of theprotons (nuclei of the hydrogen atoms in the water and oil), T₁ is thespin-lattice relaxation time, M₀ is the maximum nuclei polarization, andf is a calibration factor.

After the delay time, τ, and after the second 90 degree pulse, p₂, theFID signal is:

    F(τ)=M.sub.0 dS(X.sub.0 -vτ)(1-e.sup.-X.sbsp.1.sup./vT.sbsp.1)(1-e.sup.-τ/vT.sbsp.1)+M.sub.0 dSvτ(1-e.sup.-x.sbsp.1.sup./vT.sbsp.1),             (4)

where dSvt represents the segment of fluid entering and exiting from theinlet and outlet of the coil during time τ.

If the delay time τ equals or exceeds X₀ /v, then all the material inthe coil will contain "fresh protons" and the FID signal is at amaximum. "Fresh protons" are those that are polarized and not previouslyexposed to an RF pulse of the NMR frequency. Such exposure reduces theeffective polarization, which results in a smaller FID signal followinga second pulse of the same material.

FIGS. 8A and 8B each represent a series of measurements, where τ (thetime delay between a first pulse and a second pulse) is varied toincrease in increments of 2 milliseconds. In FIG. 8A, the FID signalamplitude reaches a maximum value at τ=26 milliseconds. This indicates aflow velocity of 1.46 meters per second for a 3.81 cm (1.5 inch) longsample coil. In FIG. 8B, the FID signal amplitude reaches a maximumvalue at τ=15 milliseconds. This indicates a flow velocity of 2.54meters per second for a 3.81 cm (1.5 inch) long sample coil.

For a given fluid, the velocity, v, can be determined by the FIDamplitude at two values of τ. Using Equation (5), two values of F(i) areobtained. One value of τ is the value representing the maximum signal,where τ≦X₀ /v , where v is a maximum expected velocity, and thus thecoil of NMR sensor 72 can be assumed to be full of fresh fully polarizedprotons. Another value of τ is at a point where the coil is less thanfull of fresh protons. The faster the velocity, the larger will be thesecond FID amplitude relative to the first. The two equations, eachrepresenting a different value of F(τ), can be solved for v. A featureof this method of determining velocity, is that measurements need bemade at only a single location in the pipeline.

For velocity analysis, fluid mixing is optional. Typically, the NMRsignal from the liquid fraction(s) is much stronger than that of the gasfraction. Thus, for unmixed fluids, the NMR analysis will substantiallyreflect the liquid velocity. Instead of mixing, it may be more desirableto maximize either the stratified or annular flow of the gas and liquidfor the velocity measurements. Thus, pre-polarizer 11 may incorporatemeans to enhance either mixing (for total flow velocity) orstratification (for liquid flow velocity). If the fluid is gas only, thegas signal will be detectable and the above-described method can be usedto determine the gas velocity.

Direct Measurement of Flow Rates

Liquid flow rates can be directly determined with NMR signalmeasurements. This method uses the following pulse sequence:

    p.sub.1 -acquisition-τ-p.sub.1 -acquisition-τ-acquisition-τ-p.sub.1 -acquisition, . . . ,

where p₁ is a 90° pulse, τ is the delay time between pulses, and"acquisition" is the time interval during which the FID NMR data isacquired from the flowing fluid. In essence, this method measures thenew polarized material that enters the NMR sensor coil between pulses.The delay time, τ, is selected to be no longer than the fluid transittime through the NMR sensor coil at the highest expected velocity, withcompensation for the signal acquisition time. Under these conditions,the amplitude of the NMR FID signal, F, may be expressed as:

    F=ρvf-g,

where ρ is the density (as measured by the hydrogen concentration in thefluids), v is the flow velocity, f is a calibration factor, and g is afactor to compensate for any residual polarization in the flowingmaterial following the pulse, p₁. Thus, the FID signal amplituderepresents a direct function of the flow rate.

For a given density, the output signal amplitude has a maximum valuewhen the sensor coil is completely full of fresh fluid at a maximumvelocity. For a lower flow velocity, the sensor coil contains old(previously measured) material as well as some new polarized material,and the signal amplitude is lower.

Thus, for direct measurement of flow rate, a first output signal isacquired when the sensor coil is known to be full of fresh fluid. Thissignal represents the maximum for that fluid. After the time delay, τ, asecond output signal is acquired. Its amplitude relative to the firstindicates the flow rate. For example, if a fluid flowing at the highestexpected velocity refills the coil during a reference delay time, τ, aseries of signals acquired after pulses with this delay time (withcompensation for the signal acquisition time) will be substantiallyconstant in amplitude, A. If the same fluid of unknown velocity has anoutput signal of only A/2 after the same reference delay time, it isknown that its velocity and therefore its flow rate are only half thatof the first. Flow rate could also be determined by taking measurementsat varying values of τ, determining which value of τ produced asubstantially constant output signal, and comparing this value of τ withthat of the reference.

This method of directly measuring flow rate, used with a mixed fluid,such as the mixed fluid of flowmeter 10, provides a total flow rate. Ifthe fluid is not mixed, the method can be used with flowmeters 70 or 90to provide a separate flow rate for the liquid constituents.

Flowmeter with Dual NMR Sensors

FIG. 9 illustrates a flowmeter 90 that uses two pre-polarizers and thustwo pre-polarization intervals. A first pre-polarizer 91 pre-polarizesthe fluid, and a first NMR sensor 92 measures the FID amplitude afterthis first polarization. A second pre-polarizer 93 is shorter than thefirst pre-polarizer 91 and partially pre-polarizes the fluid. A secondNMR sensor 94 again measures the FID amplitude. In one embodiment, thefirst and second NMR sensors and the second pre-polarizer make use of asingle, common magnet that extends over the two sensor coils and overthe separation distance between these coils.

From the two FID amplitude measurements, the separate fractional amountsof the two liquid components (oil and water) can be determined.Specifically, if the lengths of the first and second pre-polarizationflow channels are X1 and X2, respectively, the FID signals, F1 and F2,may be expressed as:

    F1=M.sub.0 fd.sub.w X.sub.0 (1-exp(-X.sub.1 /vT.sub.1w)+M.sub.0 fd.sub.oil X.sub.0 (1-exp(-X.sub.1 /vT.sub.1oil))                    (5)

    F2=M.sub.0 fd.sub.w X.sub.0 (1-exp(-X.sub.2 /vT.sub.1w)+M.sub.0 fd.sub.oil X.sub.0 (1-exp(-X.sub.2 /vT.sub.1oil)),                   (6)

where d_(w) and d_(oil) are the densities for water and oil, T_(1w) andT₁ oil are the spin-lattice relaxation time for water and oil,respectively, M₀ is the maximum polarization, and f is the calibrationfactor.

Values of T_(1w) and T₁ oil can be obtained by measurements of themixture of oil and water and used as calibration factors. The followingtable set out T₁ oil values for several crude oils. Two T₁ values arepresent and these values range from about 0.464 to 0.862 seconds for thelonger components and from about 0.077 to 0.114 seconds for the shortercomponents.

    ______________________________________                                        Crude Oil Samples                                                             T.sub.1 A(ms)  B       C    Da    Ta   Lan   Do                               ______________________________________                                        *T.sub.11 (ms)                                                                        489    795     558  477   862  479   464                              *T.sub.12 (ms)                                                                         77    101     114  101   124   93    92                              ______________________________________                                         *T.sub.11  and T.sub.12  are longer and shorter components of T.sub.1,        respectively.                                                            

The T_(1w) value for water is typically about 2.2 seconds, though it canbe much lower if the water contains paramagnetic ions. Equations (5) and(6) can be solved for d_(w) and d_(oil), and hence used to derivefractional amounts of water and oil.

The gas fraction may be determined in a manner similar to that describedabove for flowmeter 10. For flowmeter 90, this method uses the total FIDamplitude from the first NMR sensor 92 together with pressure andtemperature measurements from sensors 97 and 98.

Fluid velocity can be determined using NMR sensor 92 and a pulsesequence and the method described above for flowmeter 70. As statedabove, the fluid velocity measurement approximates the liquid velocity.If liquid fractions and liquid velocity are known, separate flow ratesfor the water and oil can be calculated. The flow rate, r, is theproduct of density and velocity.

One of the magnets of sensor 92 or 93 may provide a gradient field fordetermining separate flow velocities as described below in connectionwith FIG. 10.

Separate Flow Velocities From Gradient Field

FIG. 10 illustrates a method of measuring separate flow velocities forgas and liquid. This method involves using a pre-polarizer followed by asensor magnet, which provides a gradient magnetic field in the sensor.For example, for the flowmeter 10 of FIG. 1, the magnet in sensor 12 maybe used in this manner. Although the following description is in termsof NMR sensing, the same concepts can be used for ESR sensing.

U.S. Pat. No. 4,536,711, to King and Riewarts, entitled "Method andApparatus for Measuring Flow in a Pipe or Conduit", and incorporatedherein by reference, describes a method of using NMR or ESR sensing anda gradient magnetic filed to measure flow velocity. As a segment offlowing fluid moves through the gradient field, the frequency of the NMRsignal from the fluid changes. If the field gradient is linear, thefrequency change is linear. The frequency change at a time, τ, followingan NMR excitation pulse, p, and is proportional to the velocity of thefluid. For example, a fluid having velocity, v, will have a frequencyshift through the gradient field at time τ that is half that of a fluidhaving a velocity of 2v.

For gradient field velocity measurements, a pulse sequence appropriatefor acquiring a Hahn echo NMR signal may be used. An FID signal may alsobe used to acquire velocity data.

As illustrated in FIG. 10, by using a fast Fourier transform, the NMRsignal data may be represented as a function of frequency. If the NMRsignal is from fluid flowing in a gradient field, the frequency spectrareflects the flow velocity. For example, if the magnetic gradient causesa shift of 1.0 khz in the frequency of the NMR signal from liquidsmoving at 5 meters per second, then the NMR signal from gas moving at 25meters per second would be centered on a frequency shift of 5 khz.Although the gas signal is typically of lower amplitude than the liquidsignal, with sufficient resolution, each of these signals can beseparately detected and measured. The frequency shift provides the flowvelocity while the area under the spectra peak provides the flow densityin terms of hydrogen nuclei.

For gradient field measurements, stratified or annular flows arepreferable, and the flowmeter may contain means for enhancing thesetypes of flow. The amplitude of each spectral component is proportionalto the concentration of nuclei in an associated liquid or gasconstituent flowing at the same velocity. The frequency locationindicates the flow velocity. The separation between spectral componentspermits the higher flow velocity gas signal to be detected separatelyfrom that of the slower liquid.

The gradient field method is applicable to ESR output signals, exceptthat the amplitudes represent the density of unpaired electrons ratherthan nuclei. Also, the ESR signal would be produced only by a crude oilconstituent or other constituent with favorable ESR properties, and thevelocity would reflect total velocity if the fluid is mixed and liquidvelocity if the fluid is not mixed.

Flowmeter with ESR Sensor

FIG. 11 illustrates an ESR flowmeter 110, where ESR signals are used tomeasure the oil cut in a flowing fluid. The detected ESR signalamplitude, multiplied by a calibration factor for the apparatus and forthe type of oil, provides a measure of the oil fraction.

A ESR sensor 111 (composed of a magnet 111a to provide a field H₀ acrossthe flow line in the sensor and a resonator 111c tuned to the ESRfrequency) and detector 112 are used to obtain an ESR signal whoseamplitude is proportional to oil cut. The resonator 111c may be a coiland tuning capacitor or an appropriate cavity, such as a TE₁₀₂ mode,tuned to the ESR frequency. The flow pipe in the sensor 111 is ofnon-conductive material.

As explained above in connection with FIG. 1, different crude oils havedifferent calibration factors, which are provided to processor 113. Theprocessor 113 is appropriately programmed to calculate the crude oilfraction from the ESR signal amplitude, the dimensions of the pipe, andthe calibration factors.

By providing a gradient magnetic field for the ESR sensor, the oil flowvelocity may be determined in a manner analogous to the method describedabove for NMR. The total fluid flow velocity may also be determined byuse of a venturi such as that of FIG. 1.

Flowmeter with ESR Sensor and Gamma Ray Sensor

FIG. 12 illustrates a flowmeter 120 having an ESR sensor 121 and a gammaray sensor 122. Each has associated detector electronics 121a and 122a,respectively. ESR sensor 121 provides an ESR signal from which the oilfraction can be calculated as described above in connection with FIG.11. Gamma ray sensor 122 measures the total fluid density, from whichthe water fraction and gas fraction are calculated, using temperatureand pressure measurements from sensors 123 and 124. The liquid fractionis the difference between the total density and the gas fraction, andthe water fraction is the difference between the liquid fraction and theoil fraction. Computer 125 determines the gas, oil, water fractions fromthe ESR and gamma ray sensor data. The gamma ray processor 126 selectsand processes data from detector 122a for use by computer 126.

ESR sensor 121 can also be used to determine the liquid velocity in amanner analogous to the NMR method described above in connection withFIG. 10. In this case, the ESR signals from the fluid would be obtainedin a gradient field. If a mixer is used, there is a homogeneous blend ofliquids and gas ahead of the ESR sensor, and ESR sensor 121 provides thetotal velocity of the mixture. Alternatively, a venturi could be used tomeasure the total velocity of a mixed fluid as described above inconnection with FIG. 1.

An advantage of flowmeter 120 is that it does not require a longpre-polarization magnet--the ESR sensor magnet is sufficient. Itprovides the gas, oil, and water fractions without the need formulti-energy gamma rays, and no separate flow velocity sensor isrequired.

Other Embodiments

Although the invention has been described with reference to specificembodiments, this description is not meant to be construed in a limitingsense. Various modifications of the disclosed embodiments, as well asalternative embodiments, will be apparent to persons skilled in the art.It is, therefore, contemplated that the appended claims will cover allmodifications that fall within the true scope of the invention.

What is claimed is:
 1. A flowmeter for a fluid flowing through a flowchannel, comprising:a pre-polarizer for polarizing said fluid; a NMRsensor for providing NMR output data from the polarized fluid; an NMRdetector for powering said NMR sensor at the NMR frequency and foramplifying and detecting an NMR signal induced in said sensor from thepolarized and energized liquid flow; and a processor programmed tocalculate flow velocity by obtaining a first NMR signal emitted bycompletely polarized and energized fluid in said NMR sensor, obtaining asecond signal emitted by said fluid after a delay time less than thetime required for said fluid to flow out of said NMR sensor, andcomparing the amplitudes of the said first NMR signal and said secondNMR signal; wherein said processor is further programmed to calculate agas fraction of said fluid from the NMR signal amplitude and fromtemperature and pressure measurements.
 2. The flowmeter of claim 1,wherein said NMR detector is operable to energize said NMR sensor withone or more pulses at the NMR frequency.
 3. The flowmeter of claim 1,wherein said NMR sensor has a magnet that provides a magnetic fieldintensity through said fluid flow and has a coil tuned to the NMRfrequency and encompassing at least part of a flow line contained saidfluid.
 4. The flowmeter of claim 3, wherein an electromagnetic field ofNMR frequency produced by current in said coil is induced in said fluid.5. The flowmeter of claim 1, wherein said pre-polarizer is comprised ofa flowchannel folded in a plane and having magnets on opposing sides ofsaid plane.
 6. The flowmeter of claim 5, where said flow channel is madefrom non-ferromagnetic material.
 7. A method of determining the velocityof a fluid flowing through a flow channel, comprising:pre-polarizingsaid fluid; using an NMR sensor to provide NMR output data from saidpolarized fluid; calculating said flow velocity by obtaining a first NMRsignal emitted by completely polarized fluid in said NMR sensor,obtaining a second signal emitted by said fluid after a delay time lessthan the time required for said fluid to flow out of said NMR sensor,and comparing the amplitudes of the said first NMR signal and saidsecond NMR signal; and calculating a gas fraction of said fluid from theNMR signal amplitude and from temperature and pressure measurements. 8.A flowmeter for a flowing fluid having at least two liquid constituents,subject to temporal, spatial, and velocity changes within a flowline,comprising:a first pre-polarizer for polarizing said fluid; a first NMRsensor for providing NMR output data representing a first NMR outputsignal from said fluid after polarization by said first pre-polarizer; afirst NMR detector for powering said first NMR sensor at the NMRfrequency and for amplifying and detecting an NMR signal induced in saidsensor from the polarized liquid flow; a second pre-polarizer forre-polarizing said fluid, said second polarizer providing a polarizationtime for the flowing fluid; a second NMR sensor for providing NMR outputdata representing a second NMR output signal from said fluid afterpolarization by said second polarizer; a second NMR detector forpowering said second NMR sensor at the NMR frequency and for amplifyingand detecting an NMR signal induced in said sensor from said polarizedliquid flow; means for ensuring that the NMR signal from the second NMRsensor is from the same segment of fluid as the NMR signal from thefirst sensor; and a processor programmed to calculate the separatefractions of said liquid constituents from said NMR output data, usingknown values of NMR relaxation times of said liquid constituents.
 9. Theflowmeter of claim 8, wherein said processor is further programmed tocalculate a gas fraction of said fluid from total fluid density, asdetermined by NMR response data, and from temperature and pressuremeasurements.
 10. The flowmeter of claim 8, wherein said pre-polarizeris comprised of a flowchannel folded in a plane and having magnets onopposing sides of said plane.
 11. The flowmeter of claim 8, wherein saidmeans is accomplished by a shorter length of said second pre-polarizeras compared to the length of the first.
 12. The flowmeter of claim 8,wherein said second pre-polarizer is a continuation of the magneticfields of said first sensor and said second sensor.
 13. The flowmeter ofclaim 8, further comprising a mixer for mixing said fluid and a venturifor measuring the total velocity of the mixture.
 14. The flowmeter ofclaim 8, wherein said processor is further programmed to calculate flowvelocity by obtaining a first NMR signal emitted by completely polarizedfluid in said first NMR sensor, obtaining a second signal emitted bysaid fluid after a delay time less than the time required for said fluidin said NMR sensor to flow out of said sensor, and comparing theamplitudes of the said first NMR signal and said second NMR signal. 15.The flowmeter of claim 8, further providing means for providing agradient magnetic field in at least one of said NMR sensors and whereinsaid processor is further programmed to provide frequency spectrum datafrom said NMR data, and to determine the velocity of said liquid fromsaid frequency spectrum data.
 16. A method of determining the fractionalconstituents of a flowing fluid having at least two liquid constituents,subject to temporal, spatial, and velocity chances within a flowline,comprising:using a first pre-polarizer to polarize said fluid; using afirst NMR sensor/detector to provide NMR output data representing afirst NMR output signal from said fluid after polarization by said firstpre-polarizer; using a second pre-polarizer to re-polarize said fluid,said second polarizer providing a different polarization time for saidfluid; using a second NMR sensor/detector to provide NMR output datarepresenting a second NMR output signal from said fluid afterpolarization by said second polarizer, said NMR output data from saidsecond NMR sensor being acquired after a delay from the acquisition ofdata from said first NMR sensor; ensuring that the NMR signal from thesecond NMR sensor is from the same segment of fluid as the NMR signalfrom the first sensor; and calculating the separate fractions of saidliquid constituents using said NMR output data and known NMR relaxationtimes of said liquid constituents.
 17. The method of claim 16, whereinsaid ensuring step is accomplished by a shorter pre-polarization time ofsaid second pre-polarizer as compared to that of the first.
 18. Aflowmeter for directly measuring the flow rate of a fluid flowingthrough a flow channel, comprising:a pre-polarizer for polarizing saidfluid; an NMR sensor for providing an NMR output data from saidpolarized fluid; an NMR detector for powering said first NMR sensor atthe NMR frequency and for amplifying and detecting an NMR signal inducedin said sensor from the polarized liquid flow; means for detecting theflow velocity of said fluid; and a processor programmed to directlymeasure flow rate from stored reference NMR response amplitude data,from measured NMR response amplitude data, and from the velocity of saidfluid; wherein said processor is further programmed to calculate a gasfraction of said fluid from the NMR signal amplitude and fromtemperature and pressure measurements.
 19. The flowmeter of claim 18,wherein said means for detecting the flow velocity is a processorprogrammed to calculate said flow velocity by obtaining a first NMRsignal emitted by completely polarized and energized fluid in said NMRsensor, obtaining a second signal emitted by said fluid after a delaytime less than the time required for said fluid to flow out of said NMRsensor, and comparing the amplitudes of the said first NMR signal andsaid second NMR signal.
 20. The flowmeter of claim 18, wherein said NMRdetector is operable to energize said NMR sensor with one or more pulsesat the NMR frequency.
 21. The flowmeter of claim 18, wherein said meansfor measuring flow velocity is a venturi.
 22. The flowmeter of claim 18,wherein said processor is programmed to directly determine flow rate byuse of a first NMR signal obtained with said NMR sensor filled withfully polarized flowing fluid, a second NMR signal obtained after a timedelay that is short compared to the time required for the fluid in saidsensor to flow out of said sensor, and by determining the amplituderatio of said second NMR signal to said first NMR signal, andmultiplying the amplitude of said first NMR signal by said ratio. 23.The flowmeter of claim 22, wherein said multiplying step is followed bymultiplication by a calibration factor based on the size of said sensorand properties of said fluid.
 24. A method of directly measuring theflow rate of a fluid flowing through a flow channel, comprising thesteps of:pre-polarizing said fluid; using an NMR sensor/detector toprovide NMR output data from said polarized fluid; measuring thevelocity of said fluid; calculating said flow rate from stored referenceNMR response amplitude data, from measured NMR response amplitude data,and from the velocity said fluid; and calculating a gas fraction of saidfluid from the NMR signal amplitude and from temperature and pressuremeasurements.
 25. The method of claim 24, wherein said measuring step isperformed by calculating said flow velocity by obtaining a first NMRsignal emitted by completely polarized fluid in said NMR sensor,obtaining a second signal emitted by said fluid after a delay time lessthan the time required for said fluid to flow out of said NMR sensor,and comparing the amplitudes of the said first NMR signal and saidsecond NMR signal.
 26. The method of claim 24, wherein said measuringstep is performed with a venturi.
 27. The method of claim 24, whereinsaid flow rate is calculated by use of a first NMR signal obtained withsaid NMR sensor filled with fully polarized flowing fluid, and a secondNMR signal obtained after a time delay that is short compared to thetime required for the fluid in said NMR sensor to flow out of saidsensor, and determining the amplitude ratio of said second NMR signal tosaid first NMR signal, and multiplying the amplitude of said firstsignal by said ratio.
 28. The method of claim 27, wherein saidmultiplying step is followed by multiplication by a calibration factorbased on the size of said sensor and properties of said fluid.